What stores all messages on a disk until delivery processing is completed?

The submission queue is responsible for storing outgoing messages until they are processed for delivery. When an email is sent from a user, that message is placed in the submission queue, where it awaits further action by the transport components of Exchange Server. The submission queue allows for the organization and management of messages as they are prepared for delivery, ensuring that they are appropriately processed and routed to their destination.

In a typical Exchange Server scenario, once the message resides in the submission queue, it is eventually moved to the transport queue for further handling, where it then follows the necessary routing steps to reach the recipient. This sequential processing is essential for the efficient handling of email traffic within the server architecture.

The other options involve different stages and components of email management, such as the mailbox database, which stores users' mail, or the transport queue, which deals with routing messages. However, the key function of holding messages specifically for delivery processing is attributed to the submission queue.